To provide solar cell modules with enhanced conversion efficiency and long-term reliability over 20 to 30 years or even longer, a number of reports and proposals relating to encapsulants were made in the art. From the standpoint of efficiency enhancement, the silicone material is reported to be superior in internal quantum efficiency due to light transmittance at wavelength of about 300 to 400 nm, as compared with the ethylene-vinyl acetate copolymer (EVA) which is currently the mainstream of encapsulant (see Non-Patent Document 1, for example). In fact, an experiment to compare the output power of solar modules using EVA and silicone material as encapsulant is reported (see Non-Patent Document 2, for example).
Originally, the use of silicone material as encapsulant was already implemented in the early period of 1970s when solar cell modules for spacecraft were fabricated. Historically, in the stage when solar cell modules for ground applications were manufactured, the silicone material was replaced by EVA because the silicone material had outstanding problems including material cost and workability for encapsulation whereas the EVA was inexpensive and supplied in film form. Recently, the efficiency enhancement and long-term reliability of solar cells are highlighted again. Accordingly, the properties of silicone material as encapsulant, for example, low modulus, high transparency and weather resistance are considered valuable again. Several encapsulating methods using silicone material are newly proposed.
For example, Patent Document 1 discloses encapsulation using a sheet of organopolysiloxane-based hot melt material. However, it is difficult to work the polysiloxane into a sheet while maintaining high transparency. When the polysiloxane is shaped into a sheet of about 1 mm thick, for example, only a particular shaping technique such as casting or pressing is applicable due to the “brittleness” of the material. This shaping technique is unsuitable for mass-scale production. To ameliorate the brittleness, a filler may be admixed with the polysiloxane. Filler loading can improve moldability at the sacrifice of transparency. Patent Document 2 discloses that interconnected solar cells are positioned on or in a liquid silicone material coated on a substrate, using a multi-axis robot. The liquid silicone material is then cured, thereby achieving encapsulation without trapping air bubbles. Further, Patent Document 3 proposes that a solar cell is placed in vacuum, and the components are compressed using a cell press having a movable plate, thereby achieving encapsulation without trapping air bubbles. In these patent documents, however, no reference is made to the treatment of the solar cell module at its edge face. When silicone is used, its moisture permeability leaves a concern about the ingress of moisture. Since either of these methods differs significantly from the conventional methods of encapsulating solar cells, there is a possibility that the currently available mass-production systems cannot be used.
Patent Document 4 discloses a method of sealing a solar cell module by placing a sealing compound, a solar cell element, and a liquid silicone material on a glass substrate, then laying a back side protection substrate thereon to form a precursory laminate, and compression bonding the laminate in vacuum at room temperature. This method may be applicable with difficulty to the manufacture of solar cell modules of practical size.
Also, Patent Document 5 discloses a method of sealing a double glazed unit or solar cell panel by placing a sealing composition between peripheral bands of glass pieces in thickness direction, placing an EVA or similar resin inside the sealing composition, and heat compression bonding in vacuum. With this method, the molten EVA can be squeezed out of the peripheral bands of glass pieces in the heat compression bonding step, interfering with the adhesion of the sealing composition to the glass pieces.
Citation ListPatent Document 1:JP-A 2009-515365 (US 20080276983)Patent Document 2:JP-A 2007-527109 (US 20060207646)Patent Document 3:JP-A 2011-514680 (US 20110061724)Patent Document 4:WO 2009/091068Patent Document 5:JP-A 2011-231309Non-Patent Document 1:S. Ohl, G. Hahn, “Increasedinternal quantum efficiency ofencapsulated solar cell byusing two-component silicone asencapsulant material”, Proc.23rd, EU PVSEC, Valencia (2008),pp.2693-2697Non-Patent Document 2:Barry Ketola, Chris Shirk,Phillip Griffith, Gabriela Bunea,“Demonstration of the benefitsof silicone encapsulation of PVmodules in a large scale outdoorarray”, Dow Corning Corporation